COLUMBIANA, Alabama -- A neuropsychologist who examined Bart Wayne Johnson testified today that he diagnosed Johnson with having an acute psychotic disorder.

Outwardly, Johnson appears normal, but he kept many things built up inside that developed over his lifetime, said Dr. Charles Golden of South Florida, testifying for the defense.

Johnson suffered from insomnia, and grew up in a disfunctional family with a mother and a step-father who abandoned him, Golden testified.

Johnson, 31, is charged with capital murder in the December 2009 shooting death of Pelham police officer Philip Davis, 33, during a traffic stop on Interstate 65 in Pelham.

Golden said in his testimony that tests he performed on Johnson showed that he was someone "with the capacity of losing it and exploding" under the right set of circumstances.

Authorities say Johnson, a pharmacist, was on his way back from the Gulf Coast where he helped open a new store when he shot Davis around midnight after Davis pulled him over and wrote him a speeding ticket.

Golden testified that Johnson did not remember much about what happened when he was pulled over.

Johnson said he had not gotten any sleep at all while on the trip and had been taking Imitrex, which can exacerbate anxiety, for migraine headaches. Johnson did not know how many pills he had taken, and he had stopped at McDonald's, bought a Coke and poured rum in it, according to Golden.

Golden also said Johnson told him that before being stopped by Davis, he saw blue lights on the interstate and had in his mind that police were conducting a sting operation to get him. Johnson felt at one point like he was driving into a cemetery, and called his brother to talk.

Johnson has pleaded not guilty by reason of mental disease or defect, which his attorneys say is tied to sleep disorder and migrine headaches.